Demo code for hiding custom meta with an underscore prefix

feelings.php

<?php
/*
 * Plugin Name: Feelings
 * Plugin URI: trepmal.com
 * Description: Add your feelings to your post
 * Version: 0.1
 * Author: Kailey Lampert
 * Author URI: kaileylampert.com
 * License: GPLv2 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * TextDomain: feelings
 * DomainPath:
 * Network:
 */

$feelings = new Feelings();

class Feelings {

  function __construct() {
		add_action( 'edit_form_after_title', array( &$this, 'edit_form_after_title' ) );
		add_action( 'save_post', array( &$this, 'save_box' ), 10, 2 );
		add_filter( 'is_protected_meta', array( &$this, 'is_protected_meta' ), 10, 2 );
	}

	function edit_form_after_title() {

		wp_nonce_field( 'save_feelings', 'feelings_nonce' );
		$feelings = get_post_meta( get_the_ID(), 'feelings', true );
		?><div><label><em>Currently feeling&hellip;</em> <input type="text" name="post_feelings" value="<?php echo $feelings; ?>" class="regular-text" /></label></div><br /><?php

	}

	function save_box( $post_id, $post ) {

		if ( ! isset( $_POST['feelings_nonce'] ) ) //make sure our fields have been submitted
			return;
		if ( ! wp_verify_nonce( $_POST['feelings_nonce'], 'save_feelings' ) ) //verify intent
			return;
		if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) //no auto saving
			return;
		if ( ! current_user_can( 'edit_post', $post_id ) ) //verify permissions
			return;

		$feelings = strip_tags( $_POST['post_feelings'] ); //sanitize
		if ( empty( $feelings ) )
			delete_post_meta( $post_id, 'feelings' ); //clean up
		else
			update_post_meta( $post_id, 'feelings', $feelings ); //save

	}

	function is_protected_meta( $protected, $meta_key ) {
		if ( 'feelings' == $meta_key )
			return true;
		return $protected;
	}

}